1. Field of the Invention
This invention relates to a white balance adjusting circuit for color video cameras.
2. Description of the Related Art:
White balance adjustment is one of important signal control methods for color television cameras. White balance adjustment means that the gain for each color component of a sensed image signal is adjusted so that the ratio of three primary color components (red, green and blue) in an illumination (that is, ambient light) will be 1:1:1 for a white object. The ratio of three primary color components depends upon the type of illumination. Therefore, if the white balance adjustment is not correct, the real colors of an object cannot be correctly reproduced in the picture of the object, that is, the whole picture is bluish or reddish.
So far, external-metering automatic tracking type white balance adjusting devices have been provided (for example, in Japanese Laid-Open Patent Application Nos. Sho 54-53924 and Sho 55-158792) to assure an accurate and automatic white balance adjustment. To assure an automatic white balance adjustment, these devices are designed so as to detect the ambient light for an object by means of another optical system than a photographic optical system, and control the gain for each color component of a sensed image signal according to the results of detection. Concretely, each of these devices detects 2 or 3 different color components of the ambient light by means of 2 or 3 colorimetric sensors provided with color filters mounted on their fronts, and controls the gain of a color signal amplifier for the output of an image sensor according to the ratio of the detected values.
However, the conventional devices present a disadvantage in that, if an object or its surroundings have specific colors, the incident light in the colorimetric sensors may be influenced by these colors to deteriorate the colorimetric accuracy. If a red object is photographed, for example, the colorimetric sensors receive a red color signal having a greater amount of red components than that contained in the illumination light. As the result, the white balance adjustment is so deviated as to provide an entirely bluish picture.